弹出后重复文件描述符 [英] Duplicate file descriptor after popen

问题描述

我正在使用popen在linux下执行命令，然后4个进程将使用相同的输出. 我试图再次复制文件描述符，以将其传递给每个进程. 这是我的代码:

I am using popen to execute a command under linux, then 4 process wile use the same output. I am trying to duplicate the file descriptor again to pass it to each process. here is my code:

FILE* file_source = (FILE*) popen(source_command, "r");
int fd = fileno(file_source);
fdatasync(fd);

int dest_fd[4], y, total = 4;
    for (y = 0; y < total; y++) {
        dest_fd[y] = dup(fd);
    }

实际上，如果将total设置为1则可以正常工作，在将total = 4更改后，它将不再起作用. 这个答案太接近我所需要的: 链接

actually if total set to 1 it work fin, after changing total = 4 it does not work anymore. this answer is too close to what i need: link

推荐答案

您当前的方法可能无法满足您的要求.当您仅复制文件描述符时，它们都指向同一管道-没有 data 将被复制.对于source命令发送的每个数据块，正好有一个进程要读取它.

Your current approach probably won't do what you want. When you just duplicate the file descriptors, they all refer to the same pipe - no data is going to get duplicated. For each block of data sent by the source command, exactly one process is going to read it.

如果要复制数据(如tee实用程序一样)，则需要明确地这样做:

If you want to duplicate the data (like the tee utility does), then you will need to explicitly do so:

#define TOTAL 4

int dest_fd[TOTAL];
int dest_fd_wr[TOTAL];
int y;

/* Build pipes for reading the data from the child process */
for (y = 0; y < TOTAL; y++)
{
    int p[2];

    pipe(p);
    dest_fd[y] = p[0];
    dest_fd_wr[y] = p[1];
}

/* Create a child process to handle the "tee"-style duplication */
if (fork() == 0)
{
    /* Child process */
    FILE *file_source = popen(source_command, "r");
    FILE *file_sink[TOTAL];
    char buffer[2048];
    size_t nbytes;

    for (y = 0; y < TOTAL; y++)
    {
        close(dest_fd[y]);
        file_sink[y] = fdopen(dest_fd_wr[y], "w");
    }

    while ((nbytes = fread(buffer, 1, sizeof buffer, file_source)) > 0)
    {
        for (y = 0; y < TOTAL; y++)
        {
            fwrite(buffer, 1, nbytes, file_sink[y]);
        }
    }

    _exit(0);
}

for (y = 0; y < TOTAL; y++)
{
    close(dest_fd_wr[y]);
}

/* Now have a set of file descriptors in dest_fd[0..TOTAL-1] that each have
 * a copy of the data from the source_command process. */

错误处理留给读者练习；)

Error-handling is left as an exercise for the reader ;)

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